Tau is an intracellular microtubule-associated protein that regulates microtubule dynamics, axonal transport, and neurite outgrowth. Tau promotes microtubule assembly and is believed to be responsible for establishing neuronal polarity (Avila et al, 2004; Hirokawa et al, 1996). A natively unfolded protein, the C-terminus of the tau binds to the axonal microtubules while the N-terminus binds to the neural plasma membrane. The functions of tau are modulated via sit-specific phosphorylation. While phosphorylation is necessary for functional regulation, abnormal and excessive phosphorylation, observed in pathological states such as Alzheimer's disease and TBI, renders the protein non-functional, thus disrupting micro-tubular structure and cell death.
Alzheimer's disease (AD) is an age-dependent dementia. It has been shown that abnormally hyper-phosphorylated tau is the major protein subunit of paired helical filaments (PHF) that form intracellular neurofibrillary tangles in the brain of AD patients (Grundke-lqbal, 1986a and 1986b). The deposition of hyper-phosphorylated tau is generally localized to the temporal lobe in AD.
Traumatic brain injury may be classified into two components: i) primary brain injury occurring as a result of direct impact, and ii) secondary brain injury associated with the molecular and cellular response to injury that follows primary impact (Park et al, 2008). Recently, abnormal deposition of tau was observed in in post-mortem brains obtained from patients who suffered repetitive mild TBI during their lifetime (McKee et al, 2009; Goldstein et al, 2012). More specifically, the hyper-phosphorylated tau accumulates in neurofibrillary tangles (DeKosky et al, 2010) in the deep sulci and gyri, amygdala, and hippocampi. Hyper-phosphorylated tau has been observed both intracellular and extracellular locations in traumatic brain injury.
In Canada approximately 18,000 people per annum suffer traumatic brain injury (TBI), representing 12% of all injury hospitalizations. Most of these patients (65%) are young adults, and the direct plus indirect costs to society are estimated at approximately $1 billion (Pickett et al, 2001; Corrigan et al, 2010). Growing incidence of military combat- and sports-related repetitive mild injury to the brain emphasize not only the importance of prevention and acute treatment but also the increasing need for methods that provide an assessment of disease extent and its long term neurological sequel (Jaffee & Meyer, 2009; Klimo & Ragel, 2010; DeKosky & Ikonomovic, 2010). Although termed concussion or mild TBI, the many diverse and often disabling effects render the affected individuals unable to function at normal capacity. It is important not only to establish that TBI has occurred, but also objectively quantify the extent of damage in order to provide timely patient specific care.
Hyper-phosphorylation of tau relates to the characteristic cognitive impairment in both AD and TBI. Indeed, many studies suggest a link between TBI and a higher risk of later developing AD (Van Den Heuvel et al, 2007). While there have been reports on long term predictors of moderate and severe TBI relative to neurological function and recovery, more sensitive, specific and non-invasive methods of assessment are needed, specifically for mild TBI (Bigler & Maxwell, 2012; Marshall et al, 2012). This is particularly highlighted by the fact that, in the absence of definitive imaging techniques, diagnosis largely relies on a range of symptom complex.
In mild TBI, there presently exists a dissociation between neurological impairment and MR imaging, i.e. significant symptoms with relatively normal MR imaging (Eierud et al, 2014). While serum and cerebrospinal fluid biomarkers for TBI have evolved, with a few being studied for mild injury, there is presently no definitive imaging biomarker for mild TBI (Zetterburg, 2013). Based on the above-mentioned pathological studies, tau or its hyper-phosphorylated form could well represent a target for biomarker development.
Emerging techniques in cellular imaging have shown some progress in the use of monoclonal anti-tau antibody-coated gold nanoparticle based assays for ligand-based visualization of tau in vitro (Neely et al, 2009) and ligand-based radio-active tracers for in vivo visualization of tau in tau transgenic mice brains (Fodero-Tavoletti et al, 2011), in the context of Alzheimer's disease research.
However, generating antibodies against tau remains a challenge for the following reasons. First, tau is in a natively unfolded conformation state (Jeganathan et al, 2008). Second, it is poorly immunogenic most likely due to the fact that it has a highly conserved self-antigenic nature. While high-affinity monoclonal antibodies against tau have been generated in tau-knockout mice (Selenica et al, 2014), these have limited applications due to their large size in crossing the neuronal barriers or safety concerns for in vivo use.
The lack of specific and safe targeting agents for imaging of TBI as well as AD is an obstacle in early diagnosis. As a result, there is no mechanism in the art to visualize tau abnormality using magnetic resonance (MR) imaging.